1. Field of the Invention
This invention relates to a porous hollow fiber membrane, a method for the production thereof, and an oxygenator using the hollow fiber membrane. More particularly, this invention relates to a porous hollow fiber membrane possessing a high gas-exchange capacity and, at the same time, offering a large available membrane area for the exchange of gas, a method for the production thereof, and an oxygenator using the hollow fiber membrane. Still more particularly, this invention relates to a porous hollow fiber membrane which, no matter whether the oxygenator to be used may be adapted to pass blood inside or outside the hollow fiber membrane, refrains from inflicting damage to the blood cell components or aggravating pressure loss, exhibits high efficiency in establishing gas-liquid contact, suffers from no blood plasma leakage over a protracted service, and manifests a high gas-exchange capacity, a method for the production thereof, and an oxygenator using the hollow fiber membrane.
2. Description of the Prior Art
Generally in the surgical operation of the heart, for example, an oxygenator of hollow fiber membrane is used in the extracorporeal circulation system for the purpose of leading a patient's blood out of his body and adding oxygen to and removing carbon dioxide gas from the blood. The hollow fiber membranes available for the oxygenator of this nature fall under two kinds; homogenous membranes and porous membranes. The homogeneous membranes attain movement of a gas by the molecules of the permeating gas being dissolved and dispersed in the membrane. These homogeneous membranes are represented by silicone rubber (commercialized by Senkouika Kogyo under trademark designation of "Mella-Silox," for example). In the homogeneous membranes, the silicone rubber membrane is the only product that has been heretofore accepted as practicable from the standpoint of gas permeability. The silicone rubber membrane is not allowed to have any smaller wall thickness than 100 .mu.m on account of limited strength. Thus, it has a limited capacity for permeation of gas and it is particularly deficient in the permeation of carbon dioxide gas. Moreover, the silicone rubber has a disadvantage in that it is expensive and low in fabricability.
By contrast, in the porous membranes, since the micropores possessed by the membrane are notably large as compared with the molecules of a gas to be permeated, the gas passes through the micropores in the form of volume flow. Various oxygenators using a microporous polypropylene membrane have been proposed. It has been proposed, for example, to produce porous polypropylene hollow fibers by melt spinning polypropylene through hollow fiber producing nozzles at a spinning temperature in the range of 210.degree. to 270.degree. C. at a draft ratio in the range of 180 to 600, then subjecting the resultant hollow threads of polypropylene to a first heat treatment at a temperature not exceeding 155.degree. C., stretching the heated hollow threads by a ratio in the range of 30 to 200% at a temperature not exceeding 110.degree. C., and thereafter subjecting the stretched hollow threads to a second heat treatment at a temperature exceeding that of the first heat treatment and not exceeding 155.degree. C. (Japanese Patent Publication SHO 56(1981)-52,123). These porous hollow fibers obtained by the method just mentioned are physically caused to form micropores therein by the hollow threads of polypropylene being stretched. These micropores, therefore, are linear micropores extending substantially perpendicularly horizontally relative to the wall thickness proportionately to the degree of stretching while forming cracks in the axial direction of the hollow fiber. Thus, they have a cross section of the shape of a slit. Further, the micropores run substantially linearly and continuously through the wall thickness and occur in a high void ratio. The porous hollow fibers, therefore, have a disadvantage in that they have high permeability to steam and, after a protracted use for extracorporeal circulation of blood, they suffer from leakage of blood plasma.
As a porous membrane incapable of blood plasma leakage, for example, there has been proposed a porous polyolefin hollow fiber membrane which is produced by mixing a polyolefin, an organic filler uniformly dispersible in the polyolefin in the molten state thereof and easily soluble in a liquid extractant to be used, and a crystal seed forming agent, melting the resultant mixture, discharging the molten mixture through annular spinning nozzles and, at the same time, introducing an inert gas into the inner cavities of the spun tubes of the molten mixture, causing the resultant hollow threads to contact a cooling and solidifying liquid incapable of dissolving the polyolefin thereby cooling and solidifing the hollow threads, then bringing the cooled and solidified hollow threads into contact with a liquid extractant incapable of dissolving the polyolefin thereby extracting the organic filler from the hollow threads (Japanese Patent Application SHO 59(1984)-210,466). The polypropylene hollow fiber membrane which, as one species of the hollow fiber membranes, is produced by using as a cooling and solidifying liquid a specific cooling and solidifying liquid heretofore favorably utilized on account of the ability thereof to dissolve the organic filler does not suffer from blood plasma leakage because the pores formed therein are small in diameter and complicated in pattern of channel. Since this membrane has a small pore density per unit area, it has a possibility of exhibiting a gas-exchange capacity insufficient for the membrane to be used effectively in an oxygenator. It also has another possibility that the low molecular component of the polyolefin will mingle into the cooling and solidifying liquid capable of dissolving the organic filler and eventually adhere to the inner wall of the cooling bath tube and cause deformation of the shape of hollow fiber with elapse of time.
To overcome the impact of such a drawback as mentioned above, there has been proposed a porous polyolefin hollow fiber membrane which is produced by mixing polypropylene, an organic filler uniformly dispersible in the polypropylene in the molten state thereof and readily soluble in a liquid extractant to be used, and a crystal seed forming agent, melting the resultant mixture and discharging the molten mixture through annular spinning nozzles into hollow threads, allowing the hollow threads to contact a liquid made of the organic filler or a similar compound thereby cooling and solidifying the hollow threads, then bringing the cooled and solidified hollow threads into contact with a liquid extractant incapable of melting the propylene thereby extracting the organic filler from the hollow threads (Japanese Patent Application SHO 61(1986)-155,159). The hollow fiber membrane produced by this method is free from the drawbacks described so far. During the course of the cooling, however, the organic filler or the cooling and solidifying liquid remains locally on the outermost surface of hollow fibers before these hollow fibers are thoroughly cooled and solidified and the compositional proportion of polypropylene is lower in the outermost surface than elsewhere in the entire wall thickness and, as a result, the pores in the outer surface of hollow fiber are large and the propylene particles are interconnected in the pattern of a network and distributed in a heavily rising and falling state. The hollow fibers of this nature pose no problem whatever when they are used in an oxygenator of the type adapted to effect addition of oxygen to blood and removal of carbon dioxide gas therefrom by flowing the blood inside the hollow fibers and blowing an oxygen-containing gas outside the hollow tubes.
When the hollow fibers are used in an oxygenator of the type adapted to effect the same functions by flowing blood outside the hollow fibers and blowing the oxygen-containing gas inside the hollow fibers, however, they have a disadvantage that the aforementioned behavior of the outer surface inflict damage to the blood cell components and aggravate the pressure loss. The hollow fiber membrane, without reference to the type of oxygenator, has a disadvantage that the work of assembling the hollow fibers into the oxygenator neither proceeds efficiently nor produces a desirable potting because the adjacent hollow fibers coalesce.
In the case of the oxygenator which is formed of the porous hollow fiber membranes obtained as described above and is operated by circulating blood outside the hollow fiber membranes and blowing an oxygen-containing gas inside the hollow fiber membranes, if the gaps between the adjacent hollow fibers are narrow and substantially uniform in width throughout the entire length of hollow fibers, the air or the oxygen-containing gas is liable to stagnate easily in these gaps because of the hydrophobicity of the hollow fiber membranes. If the stagnation of the air or the oxygen-containing gas or the so-called phenomenon of air trap arises in the gaps between the adjacent hollow fibers, it impairs the flow of blood and entails a disadvantage that the clusters of the entrapped air or oxygen-containing gas obstruct the blood from gaining access to the air or oxygen-containing gas through the hollow fiber membranes, lend themselves to descreasing the available membrane area, and degrade the oxygenator's gas-exchange capacity.
An object of this invention, therefore, is to provide an improved porous hollow fiber membrane, a method for the production thereof, and an oxygenator using the hollow fiber membrane. Another object of this invention is to provide a porous hollow fiber membrane possessing a high gas-exchange capacity and, at the same time, offering a large available membrane area for exchange of gas, a method for the production thereof, and an oxygenator using the hollow fiber membrane. A further object of this invention is to provide a porous hollow fiber membrane of polypropylene which, without reference to the type of oxygenator, refrains from inflicting damage to the blood cell components and aggravating the pressure loss, entails no blood plasma leakage over a protracted service, experiences no decline of the gas-exchange capacity due to the air trap, exhibits a high gas-exchange capacity, and warrants-favorable use in an oxygenator using the hollow fiber membrane. Yet another object of this invention is to provide a porous hollow fiber membrane possessing a smooth outer surface and defying coalescence of the adjacent hollow fibers during the assembly of an oxygenator, a method for the production thereof, and an oxygenator using the hollow fiber membrane.